1. Field of the Invention
This invention relates to optical gimbals, and more particularly to the use of an optical fiber assembly wrapped across the gimbal axes to couple an off-gimbal light source to a telescope mounted on the innermost gimbal. The invention is particularly useful in Directed Infrared Countermeasures (DIRCM) systems that employ roll-nod gimbaled pointers to receive infrared (IR) radiation that is used to detect and track missiles and to transmit one or more infrared (IR) laser beams that are used to jam the seeker of a threatening IR-guided missile.
2. Description of the Related Art
The proliferation of shoulder-launched missiles known as MANPADS for “Man-Portable Air-Defense System” and their availability to terrorists present a real threat to military aircraft and particularly commercial aircraft. Estimates of the number of attacks on commercial aircraft vary, running as high as 43 hits on civilian aircraft—with 30 of these resulting in aircraft kills and the loss of nearly 1,000 lives—since the 1970s. More than half a million MANPADS have been delivered worldwide, and many of these are unaccounted for in legitimate inventories. These missiles currently use infrared seekers to track and lock-on to the aircraft. The missiles typically have a range of 1-8 km and can reach altitudes exceeding 12,000 feet. Historically, countermeasures range from evasion to flares to active IR jamming. Directed Infrared Countermeasures systems are being developed and installed on military aircraft and are being considered for use on commercial aircraft. The DIRCM includes a gimballed pointer that must detect, verify and track the threatening missile and then emit one or several modulated IR laser beams to jam the missile's IR seeker. The gimballed pointer typically slews on two orthogonal axes to track the incoming missile.
Northrop Grumman's Large Aircraft Infrared Countermeasures (LAIRCM) system uses AN/AAR-54(V) Missile Warning System (MWS) sensors, operating at ultraviolet wavelengths to detect the weapon's exhaust plume. The LAIRCM is built on the same platform as its predecessor Nemesis but uses a laser instead of a flashlamp. The LAIRCM uses free-space optics to optically couple the laser output to the gimbal. The optical path has significant optical losses, which reduces the output power of the modulated laser. Furthermore, the air-glass interfaces of the free-space components are highly susceptible to contamination and damage, which reduces the reliability of the system. In addition, the LAIRCM system can not support an additional UV laser to counter more advanced threats because the internal optics and transmit ports do not transmit UV.
BAE Systems has demonstrated a DIRCM system that is similar to the LAIRCM except that it uses an optical fiber to couple the laser to the gimbal (BAE Systems, Nashua N.H. IRIS Paper 2001CMC02x Infrared-Transmitting Fibers for Advanced IRCM Systems Demonstrations May 2001). The implementation of an “All-Fiber Path” was intended to improve output power and reliability. However, to achieve the necessary gimbal dual-axis rotation, three discrete fiber segments are coupled to each other using custom-made fiber optic rotary joints, which are capable of 360° rotation. The two short sections of fiber cable used inside the head are of larger core diameters equal to 200 and 250 μm to prevent potential loss due to rotational misalignment of the joints.
Using multiple fiber segments inside the gimbal resulted in the elimination of several actuated mirrors and servo loops which reduced complexity and could potentially enhance system reliability. However, piping the fiber through the two gimbal axes (roll and nod) mandated the use of non-continuous fiber segments coupled optically with optical rotary joints. The optical rotary joints have insertion and extraction losses at each interface between the fibers on the input and output, optical elements (input and output face) and at each air gap (minimum of three air spaces and six anti-reflection (AR) coatings per rotary joint). Each air gap is subject to contamination and, as experienced during the demonstration, damage to the AR coatings.
In a demonstration by BAE approximately 3.3 W of laser power was used in the 3-5 μm wavelength region. It was found that even at the modest power levels used in the BAE demonstration the high peak optical power from the laser caused damage to the AR coatings. However, the fibers were undamaged and continued to transmit the laser power. The rotary joints also introduce additional loading on the gimbal torquers resulting in reduced slew rates and increased settling times. This adversely impacted system performance, especially in multiple short shot engagements. The rotary joint limits any attempt to significantly downsize the gimbal to reduce aerodynamic drag due to mechanical limitations inherent in the design of the rotary joint, fiber fittings and fiber protective sheathing which limit fiber bend radius on each side of the fiber optic rotary joint.
U.S. Pat. No. 6,873,893 pertains to a jam head that is rotatable around at least two separate axes by first and second parts. A unitary infrared transmitting glass fiber of constant core diameter passes from a laser to and through the first and the second parts to convey an energetic infrared optical signal and an exit port through which the optical signal passes. FIGS. 3(a) and 3(b) illustrate a spool structure 70 which provides for storage of the fiber coil or cable when needed. FIG. 3(a) shows the spool structure 70 with glass fiber 40 loosely wrapped around and disposed on mandrel 72 passing through open slot 76 of bars 74, the fiber being loosely wrapped around the mandrel with slack fiber being in the slots where the fiber is disposed against the upper reaches of the slots. FIG. 3(b) shows the spool structure 70 tightly wrapped around the mandrel passing through open slots 76 of bars 74, the fiber being tightly wrapped around the mandrel with the fiber having no slack and disposed around the mandrel, leaving most of each open slot vacant. In short, FIG. 3(a) shows a relaxed glass fiber in a spool structure with the fiber slack stored whereas FIG. 3(b) shows a taut glass fiber in a spool structure with the fiber slack utilized for rotation. This spool structure is a complex mechanical structure that is likely to have reliability problems and also suffers from an excess of optical fiber. This extra length translates into extra optical losses of 0.2-1 dB per meter for a laser beam guided through the fiber.